capacitor size

hi all a couple of questions that hopefully somebody can help with. I have acquired a new to me compressor that needs new capacitors as the motor will not turn under load. question is- the motor is 240v,capacitors are both 40uF about the size of a red bull tin both are wired to the same terminals (2 wire) both rated at 440volts now my thinking is shouldn't one of them be rated at 230v as I'm assuming one should be a start and the other one a run and do they have to be that big, as the same size ones are about £20 each a pop I've seen some smaller ones on the bay for about £7 each . if any one could enlighten me that would be great.

regards

paul

Paul, sounds like you have 2 capacitors wired in parallel so the motor will only be cap start or cap run type. the 440v rating is correct so don't use 230v types.

Perhaps 1 has open circuited and the single 1 is not enough to kick things away.

Check also any load lifting device is working so the motor doesn't have to overcome a pressurised cylinder when starting.

Emgee

Hi Emgee

Thanks for your reply. I'm not familiar with a load lifting device could you enlighten me further

regards

paul

In this instance, as its a compressor, there are two devices that enable the motor start without load....

There is the non-return valve on the compressore discharge/inlet to the receiver, this prevents air leaking back into the cylinder. Then there is the pressure switch assembly that incorporates a valve that vents the compressor discharge line once the switch reaches its set pressure...basically it allows the air discharging from the compressor but upstream of the nrv, to be vented to atmosphere...

The combination of both allow the motor to start without load....

Edited By John Rudd on 31/10/2016 19:34:30

Just like the exhaust lifter (or decompressor) used when kick starting or bump starting a proper motor bicycle.

Tim

230V is the RMS (root mean square) voltage. The peak voltage will be 230 * 1.4142 = 325V. The capacitor has to withstand the peak voltage with a safety factor. Anything greater than 400V rating should be OK.

Question is - does it start when the air tank is completely empty? If it does, the unloader system is most likely the fault. If it will not start, remove the drive belt (if it has one) and see if the motor starts, or check that the compressor turns freely.

Your post seems to indicate the motor does run off load. If so, when does it stop? As the tank is filling or only when the compressor tries to restart automatically?

Sorry, disagree. Without wishing to confuse the OP, DC polarised caps (electrolytics) have ratings given in DC volts, whereas non-polarised caps (for use on AC circuits) have voltage ratings given as AC values (assuming a sinusoidal waveform). So for this application, you want a cap marked "230V" or higher - and definitely not a DC polarised cap (unless it's November 5th). 275V is a common (AC) voltage rating which allows for high supply voltages.

Murray

Murray,

I never thought to disavow DC electrolytic capacitors; it seemed too obvious. Just goes to show the danger of giving electrical advice on forums like this.

As you say, if the manufacture quotes an AC voltage rating then you don't need to do any further calculations. (My last high voltage exercise was connecting 230V phase to phase with a very high voltage capacitor to try and encourage my Powerline LAN plugs work throughout the house. Target voltage rating >230*root3*root2V.)

Cheers, Colin

Hi All

Thank you for all your replies, Its now sorted. It was the non return valve that was at fault I stripped it down and fettled it up put it back together and it now works a treat. In my head scratching I came across this site which is very useful diagnosing compressor problems. Its well worth a look and saving in your favourites for future use.

http://www.about-air-compressors.com/troubleshooting-your-compressor.html

Thank you once again for those who took time out to reply

regards

Paul